During the past several years catalyst development for metathesis polymerizations has surged. Most of these catalyst systems are well-defined organometallic complexes possessing a metal-carbon double bond (a metal carbene or a metal alkyledene) that can coordinate to the alkene moiety of the monomer and can perform the ring opening, particularly of cycloolefin monomers (COMs), in a rather facile manner. Most of the metals that demonstrate remarkable activity for this phenomenon are second- or third-row, mid- to late-transition metals. Although the reason for this phenomenon has not been clearly established, many theories have been advanced, the most prevalent being that late transition metals exhibit greater robustness than other transition metals towards impurities that may be inherently present in a polymerization system, and consequently can better resist degradation by those impurities.
In the case of COMs like norbornene (NB) that possess a highly strained double bond, the ring opened product is thermodynamically favored. Therefore, it is not necessary for the catalyst to possess a metal-carbene moiety in its structure to initiate the ROMP of NB. Any complex capable of initiating metal-carbene formation in situ should perform equally well as a catalyst for the ROMP. For instance, it is well known that RuCl3.3H2O can accomplish the ROMP of NB quite effortlessly, even though there is no carbene present in the catalyst. It is hypothesized that the reaction involves as a first step, when the metal halide reacts with the monomer, the formation of a metal-carbene moiety that is responsible for further polymer propagation.
The catalysts for olefin metathesis reactions that have received, by far, the highest exposure in the literature are those designed by:
(1) Richard Schrock""s group (as reported in Bazan et al., J Am. Chem. Soc., 1990, 112, 8378; Schrock et al., J Mol. Catal., 1988, 46, 243; Feldman et al., Organometallics, 1989, 8,2260; Schattenmann et al., J Am. Chem. Soc., 1996, 118, 3, 295; Murdzek et al., Organometallics, 1987, 6, 1373; and Murdzek et al.; Macromolecules, 1987, 20, 2640);
(2) Robert Grubbs""s group (as reported in Nguyen et al.,. J Am. Chem. Soc., 1993, 115, 9858; and Nguyen et al., J Am. Chem. Soc., 1992, 114, 397; and WO98/2/4 (Grubbs et al.); and
(3) Wolfgang Herrmann""s group (as reported in Herrmann et al., Angew. Chem. Int""l. Ed. Engl., 1996, 35, 1087).
The catalysts designed by the Grubbs group and the Herrmann group are based on ruthenium metal. These ruthenium complexes exhibit much more robustness in their handling than the Schrock complex based on molybdenum. These ruthenium complexes are stable in air, and do not degrade easily even when their solutions are exposed to the atmosphere for short intervals. Furthermore, they exhibit remarkable tolerance to impurities which may be inherent in polymerization systems, for example, moisture that may be present in solvents. Nevertheless, ruthenium complexes are more expensive than molybdenum, and the synthesis of these ruthenium complexes also requires some experimental manipulations using costly chemicals. The preparation of the starting material for synthesizing the Herrmann ruthenium complex, for instance, requires refluxing RuCl3.3H2O in isoprene for a fortnight. Similarly, the Grubbs ruthenium catalyst exhibits its highest activity only when PCy3, which is an expensive phosphine, is coordinated to the metal center as an ancillary ligand.
The Schrock catalyst is a molybdenum complex, which clearly exhibits the highest ROMP activity of any complex that has been reported thus far. Although this molybdenum alkylidene complex is extremely versatile, one of its major drawbacks which does not make it commercially viable is the synthetic procedure for making it. The final product is obtained after several steps that require very stringent protocols. Furthermore, this molybdenum complex is extremely sensitive to air and/or moisture and, therefore, utmost care must be exercised in its handling. Solvents used in the experimental procedures have to be scrupulously monitored for impurities because even vestigial quantities can deactivate the catalyst.
As reported in Nakayama et al., Chemistry Letters, 1997, 861, the group led by A. Nakamura has discovered a tungsten complex which, when modified by coordination with a tridentate (O{circumflex over ( )}N{circumflex over ( )}O{circumflex over ( )}) ligand, was able to perform the ROMP of NB in such a manner that the obtained polymer was predominantly cis-oriented (greater than 98%). However, this tungsten complex was active only in the presence of a Lewis acid, i.e., a dihalo aluminum alkyl compound as shown in the following equation. There was no mention of any catalytic activity that this combination of the tungsten complex and Lewis acid may have towards other COMs. 
Recently, Herrmann et al. reported in Angew. Chem. Int. Ed., Engl., 1998, 37, 2490, that their ruthenium complexes demonstrated a higher catalytic activity when an imidazolium carbene ligand represented by the following formula was coordinated to the ruthenium metal center. 
Finally, the Grubbs group has reported in Scholl et al., Tetrahedron Letters, 1999, 40, 2247; and in Grubbs, Presentation at the Commercialization of Polymers Meeting, San Francisco, Sep. 1999, that the use of the above carbene ligand to coordinate to the central ruthenium metal in place of one of the tri-cyclohexylphosphines in the Grubbs catalyst increased the overall potency of the catalyst for the ROMP of COMs.
In most instances, several synthetic steps are required to prepare the prior art catalyst complexes. These syntheses are interesting from an academic point of view. However, the industrial use of such catalysts may not always be commercially feasible because of economic considerations.
One aspect of the invention is to provide a polymerization system that exhibits outstanding reactivity in the ring-opening metathesis polymerization (ROMP) of cycloolefin monomers (COMs), and is simple and economical. The method of the invention produces ring opened cycloolefin polymers by using molybdenum catalysts that are readily available. The polymers are obtained in good yields, are readily soluble in common organic solvents, and are predominantly cis-oriented. The use of the catalysts according to the invention does not require any sophisticated equipment for experimental manipulations. For the ROMP of cycloolefin monomers according to the method of the invention, the catalysts include MoOCl4 and other complexes such as MoOCl2(t-BuO)2 which are prepared from MoOCl4. The catalysts are used in conjunction with a Lewis acid as a co-catalyst, and a chain transfer agent for controlling the molecular weight of the obtained polymer.
Another aspect of the invention provides a system and a method for the co-polymerization of NB and DCPD in which the two monomers are blended homogeneously, resulting in a polymer product characterized by a monomodal GPC (gel permeation chromatography) peak.
In the method of the present invention, cycloolefins are polymerized by utilizing as a catalyst a complex represented by the formula MoOX2L2 described below, wherein molybdenum has a +6 oxidation state, possesses an electron count of 12, and is penta-coordinated. Although the catalyst can achieve the ring opening of COMs independently, in the method of the present invention the performance of the catalyst is greatly enhanced by the use of specific Lewis acids as co-catalysts. A chain transfer agent (CTA) that aids in chain scission is also used in the method of the invention to tailor the final polymer product to have specific properties.
Specifically, the method of the present invention for ring opening metathesis polymerization of a cycloolefin monomer uses a polymerization system that comprises:
(a) MoOX2L2 as a catalyst, wherein the catalyst is at least one represented by the formula 
xe2x80x83wherein
(1) both of the X groups are the same halogen atom selected from the group consisting of Cl and Br, and both of the L groups are the same halogen atom as X; or
(2) both of the X groups are the same halogen atom selected from the group consisting of Cl and Br, and both of the L groups are the same straight chain or branched alkyl group having 1-4 carbon atoms; or
(3) both of the X groups are the same halogen atom selected from the group consisting of Cl and Br, and both of the L groups are the same straight chain or branched alkoxy group having 1-4 carbon atoms; or
(4) both of the X groups are the same alkyl group having 1-4 carbon atoms, and both of the L groups are the same alkoxy group having 1-4 carbon atoms; or
(5) one X group is a halogen atom selected from the group consisting of Cl and Br, and the other X group together with both of the L groups constitute a tridentate ligand; or
(6) one X group is a straight chain or branched alkoxy group having 1-4 carbon atoms, and the other X group together with both of the L groups constitute a tridentate ligand;
(b) a Lewis acid as a co-catalyst, wherein the Lewis acid is at least one selected from the group consisting of an alkyl aluminum compound, an alkoxy aluminum compound, a dialkyl-halo aluminum compound, and a dihalo-alkyl aluminum compound, wherein the alkyl group and the alkoxy group have 1-6 carbon atoms and the halogen is selected from the group consisting of Cl and Br;
(c) at least one chain transfer agent; and
(d) at least one cycloolefin monomer;
wherein the molar ratio of the catalyst (a) to the monomer (d) is in the range 1:700 to 1:100,000.
Examples of the monodentate groups X or L in the MoOX2L2 catalyst are: chlorine, bromine, methyl, ethyl, n-propyl, isopropyl, butyl, t-butyl, methoxy, ethoxy, n-propoxy, isopropoxy, butoxy, t-butoxy, etc.
An example of the tridentate ligand in the MoOX2L2 catalyst is 2,6-bis(2-methyl-2-hydroxypropyl) pyridine.
The MoOX2L2 catalysts used in the polymerization system according to the invention may be synthesized, for example, by ligand substitution in a one-step synthesis from the commercially available MoOCl4 complex and the starting materials for the desired ligands. In general, the formation of the molybdenum catalysts can be completed within a day, in most cases with percent yields which are good to excellent, typically greater than 90% in most instances. The reactions are sufficiently clean with practically no side products or competing reactions occurring simultaneously. The synthesis of the complex is generally carried out at ambient temperature inside an inert atmosphere glove-box with minimum constraints.
Examples of the catalysts are: MoOCl4, MoOCl2(t-BuO)2 wherein t-Bu is tert-butyl, MoO(Me)2(t-BuO)2 wherein Me is methyl, MoO(n-Bu)2(t-BuO)2 wherein n-Bu is n-butyl, etc. MoOCl4 and MoOCl2(t-BuO)2 are preferred catalysts for use in the polymerization system according to the invention because they are readily available, are easy to use and result in high yields. The MoOCl2(t-BuO)2 complex can be prepared by reacting MoOCl4 with t-BuOK, according to the following reaction: 
The Lewis acid used as a co-catalyst may be an alkyl aluminum compound, an alkoxy aluminum compound, a dialkyl-halo aluminum compound, or a dihalo-alkyl aluminum compound, wherein the alkyl group has 1-6 carbon atoms, preferably 1-3 carbon atoms. These Lewis acids are widely available commercially. The preferred Lewis acids are alkoxy aluminum compounds, also known as aluminoxanes, which have an oligomeric structure with repeating units of aluminoxane. The aluminoxane oligomers may be linear, cyclic, or a mixture of linear and cyclic oligomers, wherein the number of repeating units is from 2 to 50, preferably 15 to 30. The oligomeric aluminoxanes may be prepared by reaction of the corresponding alkyl aluminum compounds and water, or by other methods known in the art, such as reaction of the corresponding alkyl aluminum compounds with aluminum salts, preferably aluminum sulfate, containing water of hydration.
The amount of Lewis acid in the polymerization system is from 4 to 1,000, preferably from 10 to 100 parts of Lewis acid to 10,000 parts of the monomer on a molar equivalent basis.
The chain transfer agent is not particularly limited, and may be any chain transfer agent capable of controlling the molecular weight of the polymer to be in the desired range and distribution. The chain transfer agent may be any monomer that has a readily available xcex2-hydrogen that can easily dissociate, for example, methyl methacrylate. The chain transfer agent may be also, for example, an xcex1-olefin, specifically an xcex1-olefin having from 2 to 12 carbon atoms.
The amount of the chain transfer agent in the polymerization system is from 4 to 1,000, preferably from 10 to 100 parts of chain transfer agent to 10,000 parts of the monomer on a molar equivalent basis.
The cycloolefin monomer which can be polymerized according to the method of the present invention is not particularly limited. Examples of the cycloolefin monomer include norbornene (NB) and compounds derived therefrom which contain a norbornyl group such as tetracyclododecene (TCD) and ethylidene TCD, as well as dicyclopentadiene (DCPD).
The polymerization system may contain one or more cycloolefin monomer. In one embodiment of the invention, the combination of norbornene and dicyclopentadiene is advantageously co-polymerized homogeneously to give a polymer product which exhibits a single peak in gel permeation chromatography (GPC).
The molar ratio of monomer to catalyst xe2x80x9c[M]/[C]xe2x80x9d in the polymerization system of the invention is in the range from a ratio of 1:700 to a ratio of 1:100,000 . A preferred range is from a ratio of 1:1,000 to a ratio of 1:20,000, most preferably from a ratio of 1:2,500 to a ratio of 1:25,000.